Mississippi Tornadoes: Science of the Supercells

Mississippi bore the brunt of what can only be described as an “extreme” tornado event on April 24, 2010. It is comparable in severity to the intense outbreak of February 2008, although the damage and death toll was much greater in that series, due to denser population centers affected.

A rare “high risk” tornado threat was posted by the Storm Prediction Center (SPC) hours before the event started. The forecast was excellent even the day before when a “moderate” risk was posted for much of MS and AL.

At press time, a total of 69 tornadoes had been tallied from April 24. Adjusting for duplicate reports, we can assume about 59 of these were discrete tornado events. The text book spring tornado-producing system spawned a total of 126 tornadoes (107 after duplicate adjustments) in the three day period April 22-24. This means a short march of three days produced more tornadoes than seen in the previous four months!

This is a top-five record for a single day tornado event.

So what caused this? I’ll try to explain this using my training along with commentary and data from the SPC and National Weather Service (NWS).

Two of the primary factors for tornado production are wind shear and vigorous upward vertical motion. While weather scientists continue to study severe storm formation in depth, the existing research results support the following concept. This applies most closely to strong tornadoes such as the April 24 event. Large twisters occasionally form in other conditions but these below account for most of the major historic outbreaks:

1) Pronounced wind shear, a change of wind speed and direction with height, is indicated on the vertical analysis. Fast low-level winds of 30mph or greater boost the risk.

2) A deep layer of high moisture content

3) A lifting mechanism, such as an upper level trough, surface cold front, outflow boundary, dry line (or a combination of these) acts as a focal point for storm initiation. The map below shows a deep trough at 500mb (18k foot) level.

All three of the conditions listed above were present that day. Given this, the NWS was on the money with their discussion of that morning. Here are excerpts from it below:

“..PARTICULARLY DANGEROUS SEVERE WEATHER OUTBREAK LIKELY TODAY...

“THERE ARE MANY FACTORS COMING TOGETHER THIS MORNING TO SUGGEST THE SUBSEQUENT DEVELOPMENT
OF POWERFUL AND POTENTIALLY TORNADIC THUNDERSTORMS….

…THE STORM PREDICTION CENTER (SPC) HAS PLACED NEARLY THE ENTIRE ARKLAMISS REGION IN A RARE HIGH RISK FOR SEVERE THUNDERSTORMS TODAY (ALTHOUGH A FEW COUNTIES IN SOUTHEAST MS ARE ONLY IN A MODERATE RISK). A MODERATE RISK IS A RATHER OMINOUS FORECAST FROM THE SPC...BUT A HIGH RISK IS GENERALLY ONLY ISSUED BY THEM A FEW TIMES A YEAR ACROSS THE COUNTRY WHEN PARTICULARLY DANGEROUS SITUATIONS
ARISE….

…THE MAIN INGREDIENTS FOR A VOLATILE SETUP INCLUDE A STRENGTHENING LOW PRESSURE SYSTEM PASSING A LITTLE TO OUR NORTHWEST...HIGH LOW LEVEL AND DEEP LAYER WIND SHEAR TO ORGANIZE AND ROTATE STORMS AND AID IN TORNADOGENESIS...HIGH SURFACE MOISTURE WITH DPS IN THE UPPER 60S TO LOWER 70S...AND MODERATE TO HIGH INSTABILITY DUE TO THIS
MOISTURE AND STEEP ATMOSPHERIC LAPSE RATES ALOFT”.

April 24: Note dramatic increase in moisture and winds are now 60-65mph in lowest layers (area along bottom right of chart).

A large flag represents 50knots (58mph) while a thin barb is 10knots and a short thin barb is 5knots.

The Jackson, MS morning “sounding” for April 24 (above) was provided by an ascending weather sensor tied to a balloon. It showed several severe weather indexes off of the charts from even a “regular” tornado day.

So...how do these factors get the tornado turning? After all, these winds are blowing parallel to the ground, not in a circle.

The surging and changing wind in the lowest three miles of the atmosphere is key. The data strongly suggests that as the air near the surface (lowest few thousand feet above ground) is lifted by some mechanism (front, upper trough, etc.), the horizontal “turning” caused by the varied winds is “stood” into the vertical by the developing updrafts.

Alan Moller, a retired NWS severe weather expert, uses the analogy of a garden hose. Imagine this hose spinning like a big rolling pin in the sky, parallel to the ground.

The intense updrafts then push this “hose” into a vertical configuration over time. Now you have two vertically rotating columns of air. The counter-clockwise (cyclonic) spin usually spawns the tornado while the clock-wise one becomes part of a weaker circulation or an “anticyclonic” tornado every now and then.

Back to our typical (cyclonic) tornado now developing. The prevailing theory until a few years ago was once this “tube” of rapidly spinning air transformed the thunderstorm into a rotating Supercell, also known as a mesocyclone, it would work its way to the ground with time and a tornado would form.

More recent analysis shows that while this sometimes happens, a greater majority of storms are accelerated to tornado production by a combination of the mid-level rotation and a surface-based meeting/turning of winds near the ground.

Low-level turning is enhanced along a boundary where two different air masses with generally dissimilar wind speeds and direction work together. They get the air turning and rising. This in concert with the developing mid-level circulation is often the “final straw” that brings the tornado forth.

The science of a hundred years ago would have had virtually no chance of predicting such an event so accurately.

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